15. Nuclear reaction induced by neutrons
Fast
neutron collision with a nucleus in most cases leads to a neutron
scattering , ie, to change the direction of its flight and transfer
with the core part of the energy . Possible , however, another result
of the collision : the neutron captured by the nucleus , and because
of this nuclear reaction takes place . Examples of nuclear reactions
induced by neutrons is the splitting of boron :
Boron nucleus capturing a neutron splits into the nucleus of lithium
and helium , flying at high speed. The reaction of boron with
neutrons can be observed by placing the camera in a thin layer of
cloud forest. Irradiating the chamber with fast neutrons , we shall
see in the pictures fat traces.
Boron nucleus capturing a neutron splits into the nucleus of lithium and helium, flying at high speed. The reaction of boron with neutrons can be observed by placing the camera in a thin layer of cloud forest.
We surround the neutron source material containing a lot of hydrogen , such as paraffin wax 15-20 cm diameter sphere now on the way to the camera neutrons will collide with the nuclei of carbon (A = 12) and, most significantly , with protons. However, as we explained in the previous section, the neutrons are slowed down and fall into a cloud chamber with energy, many times at its initial energy. Action paraffin will be unexpected: the number of tracks in the images, which means the number of disintegrations of boron will repeatedly increase (Fig. 398, b). Consequently, the slower neutrons, the more efficiently they are captured by nuclei and produce nuclear reactions. In addition to the neutron velocity , the efficiency with which neutrons are captured substance also depends on the kind of atoms. Watching the passage of slow neutrons through a layer of boron , we find that they are almost completely retained boron layer thickness of a millimeter . Similar experiments show that except for boron neutron slow strongest sinks are cadmium, lithium, chloro , silver, etc. In contrast, materials such as beryllium, heavy water , carbon, bismuth, absorb slow neutrons extremely weak. Strong absorption of slow neutrons by nuclei due to the lack of electrical repulsion forces ( since the neutron deprived charge ) and the existence of attractive forces between the nuclei and neutrons (see § 225). Fast neutron flies past the kernel in such a short period of time that the forces of attraction does not have time to reject it and draw into the kernel. The slower moving neutron , the more time he is under the influence of forces of attraction by the nucleus and the easier it is captured . Capture nuclei is one of the reasons why the neutrons do not exist for a long time in the free form . The second reason is the radioactivity of the neutron. Experiments show that the free neutron over time turns into a proton , thereby emitting an electron and a neutrino
24.
Key koncepts in the physics of nuclear reactions.
The
nuclei of all
atoms
can
be divided
into
two broad classes:
stable and
radioactive.
Last
spontaneously
decay,
turning into the
nuclei of other
elements.
Nuclear
transformations
can
occur
with
the stable nuclei
in
their interaction
with
each other
and
with various
microparticles.
Any
positively
charged
nucleus
and
the
magnitude
of the charge
is
determined by
the
number of protons
in
the nucleus
Z
(atomic
number). The
number of protons
and
neutrons
in the nucleus
determines
the
mass number of
the
nucleus
A.
Symbolically
kernel
is
written as:
where
X
-
symbol
of
a
chemical element.
Kernel
with
the same
charge
number
Z
and
different
mass
numbers
A
are
called isotopes.
For
example,
uranium is
found in nature
mainly
in
the
form of two
isotopes
Isotopes
have the same chemical properties and different physical . For
example, uranium isotope 23592U interact well with the neutron 10n
all energies and can be divided into two lighter nuclei . At the same
time isotope uranium 23892U divisible only when interacting with
neutrons of higher energies than 1 mega
elektronovolta
( MeV ) ( MeV 1 = 1.6 × 10-13 J) . A kernel with the same and
different Z are called isobars. While nuclear charge equal to the sum
of the charges of its constituent protons , the core mass is not
equal to the sum of individual free protons and neutrons ( nucleons)
, it is somewhat lower. This is explained by the fact that for the
nucleons in the nucleus (for the organization of the strong
interaction ) requires the binding energy E. Each nucleon ( proton
and neutron) , falling into the nucleus , figuratively speaking,
allocates a portion of its mass to form intranuclear strong
interaction , which "glues" the nucleons in the nucleus. At
the same time , according to the theory of relativity ( see Chapter
3) , between the energy E and mass m there is a relation E = mc2,
where c - speed of light in vacuum . So that the formation energy of
the nucleons in the nucleus Eb Eb · c2. These views are confirmed by
numerous experiments. The dependences of the binding energy per
nucleon E St. / A = A, A kernel of the most stable , they have a
large binding energy . It offers the possibility of obtaining energy
in the fission of a heavy nucleus into two lighter ( medium ) . Such
a nuclear fission reaction can be realized by bombarding uranium
nuclei free neutron . For example , 23592U divided into two new
nuclei : rubidiy37 -94Rb and cesium 14055Cs ( one of the variants of
uranium fission ) . Fission of a heavy nucleus is remarkable in that
in addition to new lighter nuclei , two new free neutron, which are
called secondary . Thus on each fission have to 200 MeV energy
released . It is released as kinetic energy of the fission products
and may further be used e.g. for heating water or other coolant.
Secondary neutrons in turn can cause fission of other uranium nuclei
. Formed a chain reaction , which resulted in the multiplying medium
can stand tremendous energy . This method of energy is widely used in
nuclear munitions and controlled nuclear power plants in power plants
and transportation facilities with nuclear power. In addition to the
method for producing atomic ( nuclear ) energy , there is another -
the fusion of two light nuclei into heavier nucleus . The process of
combining light nuclei can occur only when approaching initial nuclei
at a distance , where there are nuclear forces ( strong interaction )
, ie, ~ 10 - 15 m This can be achieved at very high temperatures of
about 1,000,000 ° C. Such processes are called thermonuclear
reactions.
Thermonuclear reactions in nature are the stars , and
of course the sun. In the Earth , they occur when a hydrogen bomb (
thermonuclear weapon ) , which serves as a primer for a conventional
atomic bomb , which creates conditions for the formation of
ultra-high temperatures . Controlled thermonuclear fusion has only
until the research focus. Industrial installations not, however, work
in this direction are in all developed countries , including in
Russia.
33. Division scheme of fuel in the reactor on fast neutrons. A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons. Such a reactor needs no neutron moderator, but must use fuel that is relatively rich in fissile material when compared to that required for a thermal reactor. Although it is currently (2010) uneconomic,[5] a fast neutron reactor can reduce the total radiotoxicity of nuclear waste, and dramatically reduce the waste's lifetime.[6] They can also use all or almost all of the fuel in the waste. Fast neutrons have an advantage in the transmutation of nuclear waste. With fast neutrons, the ratio between splitting and the capture of neutrons of plutonium or minor actinide is often larger than when the neutrons are slower, at thermal or near-thermal "epithermal" speeds. The transmuted odd-numbered actinides (e.g. from Pu-240 to Pu-241) split more easily. After they split, the actinides become a pair of "fission products." These elements have less total radiotoxicity. Since disposal of the fission products is dominated by the most radiotoxic fission product, Cesium 137, which has a half life of 30.1 years,[6] the result is to reduce nuclear waste lifetimes from tens of millennia (from transuranic isotopes) to a few centuries. The processes are not perfect, but the remaining transuranics are reduced from a significant problem to a tiny percentage of the total waste, because most transuranics can be used as fuel.
Fast reactors technically solve the "fuel shortage" argument against uranium-fueled reactors without assuming unexplored reserves, or extraction from dilute sources such as ordinary granite or the ocean. They permit nuclear fuels to be bred from almost all the actinides, including known, abundant sources of depleted uranium and thorium, and light water reactor wastes. On average, more neutrons per fission are produced from fissions caused by fast neutrons than from those caused by thermal neutrons. This results in a larger surplus of neutrons beyond those required to sustain the chain reaction. These neutrons can be used to produce extra fuel, or to transmute long half-life waste to less troublesome isotopes, such as was done at the Phénix reactor in Marcoule in France, or some can be used for each purpose. Though conventional thermal reactors also produce excess neutrons, fast reactors can produce enough of them to breed more fuel than they consume. Such designs are known as fast breeder reactors.
The fast reactor doesn't just transmute the inconvenient even-numbered transuranic elements (notably Pu-240 and U-238). It transmutes them, and then fissions them for power, so that these former wastes would actually become valuable.
Nuclear reactor design
Coolant
Water, the most common coolant in thermal reactors, is generally not a feasible coolant for a fast reactor, because it acts as a neutron moderator. However the Generation IV reactor known as the supercritical water reactor with decreased coolant density may reach a hard enough neutron spectrum to be considered a fast reactor. Breeding, which is the primary advantage of fast over thermal reactors, may be accomplished with a thermal, light-water cooled & moderated system using very high enriched (~90%) uranium.
All current fast reactors are liquid metal cooled reactors. The early Clementine reactor used mercury coolant and plutonium metal fuel. NaK coolant is popular in test reactors due to its low melting point. In addition to its toxicity to humans, mercury has a high cross section for the (n,gamma) reaction, causing activation in the coolant and losing neutrons that could otherwise be absorbed in the fuel, which is why it is no longer used or considered as a coolant in reactors. Molten lead cooling has been used in naval propulsion units as well as some other prototype reactors. All large-scale fast reactors have used molten sodium coolant.
Another proposed fast reactor is a Molten Salt Reactor, one in which the molten salt's moderating properties are insignificant. This is typically achieved by replacing the light metal fluorides (e.g. LiF, BeF2) in the salt carrier with heavier metal chlorides (e.g., KCl, RbCl, ZrCl4).
Gas-cooled fast reactors have been the subject of research as well, as helium, the most commonly proposed coolant in such a reactor, has small absorption and scattering cross sections, thus preserving the fast neutron spectrum without significant neutron absorption in the coolant.[citation needed]
Nuclear fuel. In practice, sustaining a fission chain reaction with fast neutrons means using relatively highly enriched uranium or plutonium. The reason for this is that fissile reactions are favored at thermal energies, since the ratio between the Pu239 fission cross section and U238 absorption cross section is ~100 in a thermal spectrum and 8 in a fast spectrum. Fission and absorption cross sections are low for both Pu239 and U238 at high (fast) energies, which means that fast neutrons are likelier to pass through fuel without interacting than thermal neutrons; thus, more fissile material is needed. Therefore it is impossible to build a fast reactor using only natural uranium fuel. However, it is possible to build a fast reactor that will breed fuel (from fertile material) by producing more fissile material than it consumes. After the initial fuel charge such a reactor can be refueled by reprocessing. Fission products can be replaced by adding natural or even depleted uranium with no further enrichment required. This is the concept of the fast breeder reactor or FBR.So far, most fast neutron reactors have used either MOX(mixed oxide) ormetal alloyfuel. Soviet fast neutron reactors have been using (high U-235 enriched) uranium fuel. The Indian prototype reactor has been using uranium-carbide fuel.
While criticality at fast energies may be achieved with uranium enriched to 5.5 weight percent Uranium-235, fast reactor designs have often been proposed with enrichments in the range of 20 percent for a variety of reasons, including core lifetime: If a fast reactor were loaded with the minimal critical mass, then the reactor would become subcritical after the first fission had occurred. Rather, an excess of fuel is inserted with reactivity control mechanisms, such that the reactivity control is inserted fully at the beginning of life to bring the reactor from supercritical to critical; as the fuel is depleted, the reactivity control is withdrawn to mitigate the negative reactivity feedback from fuel depletion and fission product poisons. In a fast breeder reactor, the above applies, though the reactivity from fuel depletion is also compensated by the breeding of either Uranium-233 or Plutonium-239 and 241 from Thorium 232 or Uranium 238, respectively.